Gases are used in many industrial processes. Certain of these processes, such as the manufacture of semi-conductor devices, require that these gases be of extremely high purity. Any impurity gases present must presently be held in the range of several parts per billion relative to the process gas (i.e. several parts of impurity to 10.sup.9 parts of process gas).
To reach this low impurity level there are employed "gas purifiers". In the semiconductor industry there are widely employed "inert gas purifiers". By this term is meant a purifier of the rare gases He, Ne, Ar, Kr, Xe and N.sub.2. Such inert gas purifiers have been described, for example, in British patents Nos. 2,127,079 and 2,127,080. See also EP patent application No. 0,365,490.
While such inert gas purifiers are very efficient in maintaining the impurity levels of their output gas at the very low values required, they eventually start to loose their ability to remove the impurity gases. Thus the process gases become less pure and the semiconductor devices produced commence to exhibit an excessive number of defects and must therefore be rejected. Such rejects can be extremely costly especially when one considers the manufacture of semiconductor devices which require the use of micron (micro-meter) or sub-micron technologies.
It is therefore essential to ensure that the impurity level of the process gas is below the required limits.
Several methods are presently known for monitoring the impurity level of these inert process gases. One method is to constantly monitor the impurity level of each impurity gas. This, however, requires extremely specialized equipment and highly trained personnel. Another method is to measure the quantity of gas which has passed through the gas purifier and, assuming a known impurity content, calculate when the gas purifier should begin to loose its purification efficiency. Unfortunately the impurity level of the gas to be purified, may vary in an unknown manner generating errors in the calculation resulting in the continued use of the purifier even though it has reached the end of its useful life. Other methods have been proposed using physical changes in the properties of the impurity gas sorbing material such as, for instance, a change of colour.
Furthermore, when for instance the gas purification takes place, by means of metallic gettering materials there may be no indication of any colour change.